"...And the only reason I'm singing you this song now is cause you may know somebody in a similar situation, or _you_ may be in a similar situation, and if you're in a situation like that there's only one thing you can do and that's walk into the shrink wherever you are, just walk in, say "Shrink, You can get anything you want, at Alice's restaurant.". And walk out. You know, if one person, just one person does it they may think he's really sick and won't take him. And if two people, two people do it, in harmony, they may think they're both faggots and they won't take either of them. And three people, three, can you imagine, three people walking in sing a bar of Alice's Restaurant and walking out. They may think it's an organization. And can you imagine fifty people a day, I said fifty people a day walking in sing a bar of Alice's Restaurant and walking out. And friends they may thinks it's a movement."

This also appeared in RobertHeinlein's TheNumberOfTheBeast - when someone wanted something put in permanent storage, they said "I tell you three times" to the computer, and it saved three separate copies of it to make sure it was never corrupted.

It seems logical to me, that once you have three occurrences of an event you can do something like a majority vote: What happens most (all) of the time? If something happens twice, with different results, you build the rule: It's randomly once this, once that. If it's the same result, you still think it might be randomly, but develop the thesis, it might follow a pattern. The third event then "proves" that thesis. -- MaxVoelkel

The first time is happenstance.
The second time is coincidence.
The third time is enemy action.
-- Auric Goldfinger

We have a similar motto in mathematics: If it works once, it's a trick. If it works twice, it's a technique. If it works three times, it's a theorem. -- CameronSmith

It takes three legs to make a tripod or to make a table stand,
And it takes three wheels to make a vehicle called a tricycle
And every triangle has three corners,
Every triangle has three sides,
No more, no less,
You don't have to guess
That it's three
Can't you see?
It's a magic number

Even though we have GangOfFour not GangOfThree? ;-) but I still believe three is a wonderful number. Explanation for wonderfulness of this very number is simple and natural.

Most of the natural phenomena can be explained in three dimensions or, most of the natural phenomena for which we know the behavior(s) in all the three dimension are predictable, thus generalizable. Still for some gaining predictability calls for considering the fourth dimension.

As we know physics is study of nature. On the same lines I can say SoftwarePhysics is study of nature of software systems. That we know as of now is not even as mature as was ClassicalPhysics? in 18th century, so realization of fourth dimension is still miles away.

If you think of this in an abstract mathematical way, it's quite clear why this rule holds. Finding similarities is analogous to drawing a line. If you have only two examples then you can always draw a line through it. However with three points you can check whether the similarities you're deducing between systems is actually something that holds (IsAPattern?). -- ChristophePoucet?

I'd like to point out that the above reasoning is entirely fallacious, relying on ProofByAnalogy?, and as we all know ProofByAnalogyIsFraud?. Ex: [sarcasm mode] Obviously, we need to use a best fit quadratic or a best fit plane instead of a line. Those require 3 points to define, and a fourth point can reveal if it actually fits the pattern. [/sarcasm mode]

(Additionally, there are other useful states/substates, such as "blocked", "abandoned", etc. However, making distinctions between levels of doneness other than the above is asking for trouble - see NinetyPercentDone).

Surprised that the HolyTrinity? hasn't been listed. Or the Three Wise Men for that matter.